Spray drying is a widely used and well-established process across many industries, including food, pharmaceuticals, cosmetics, and chemicals. The process offers numerous benefits, such as reducing microbial growth, minimizing enzymatic degradation reactions, and significantly reducing the final volume of the product. This makes spray drying an important tool for production of high-quality, stable powders suitable for storage, application, and transportation (Dantas et al., 2023).
In the spray drying process, a solution, suspension, or emulsion is atomized into fine droplets, which are then exposed to a heated carrier, such as air or superheated steam, in the drying chamber (Dantas et al., 2023; Sobulska et al., 2022; Walton & Mumford, 1999). As a result of this interaction, the droplets rapidly lose moisture, which leads to the conversion of the liquid into solid. The fundamental criterion of spray drying processes classification is the method by which the atomized material comes into contact with the drying medium. This can take form of a co-current, counter-current, or mixed flow mode. Drying systems that operate in co-current flow are often preferred in the industry due to simple construction setup and easy control of the process. Unlike counter-current dryers, co-current systems enhance safety for thermosensitive materials by preventing contact of the dry material with hot inlet air (Zbicinski & Piatkowski, 2009a).
Most spray-dried products can be divided into three main categories, according to the morphology of their particles: skin-forming materials, porous materials, and materials with crystalline structure. Porous materials, also known as agglomerates, form particles bound by submicron dust or a binder, typically with a regular, highly spherical shape and with minimal surface irregularities. The drying process involves gradual solvent evaporation from within the particles, facilitated by the highly porous structure. This prevents significant pressure build-up inside the particle and thus avoids deformation or expansion, so blowholes and cratering are not commonly observed features. Porous materials include silica, colloidal carbon, cocoa, and detergents (Zbiciński & Kwapińska, 2003).
Materials with crystalline structure exhibit highly ordered arrangements of atoms or molecules, forming solid structures composed of large individual crystal nuclei bound together in a microcrystalline phase. The morphology, shape, and size of particles depend on the type of substance and drying conditions. Both solid and hollow particles occur. In some cases, significant internal pressure can develop from the evaporation of solvents within a particle. This can lead to disruptions, resulting in the formation of craters and secondary nucleation centers. Examples of materials with crystalline structures include sodium chloride, sodium carbonate, zinc sulfate, sodium pyrophosphate, sodium benzoate, and sodium formate (Walton & Mumford, 1999).
In skin-forming materials, drying initially occurs on the droplet's surface, increasing local viscosity and leading to the formation of a thin, hard outer layer known as a 'skin' or 'shell.' This layer is composed of a continuous, non-liquid phase, either polymeric or sub-microcrystalline. These materials typically form spherical particles with smooth surfaces and may be either hollow or solid. Hollow particles, are susceptible to collapse after drying, unlike solid particles, which retain their structural integrity. Surface-active molecules facilitate skin development by accumulating at the phase interface. An increase in temperature, which leads to a higher evaporation rate, also accelerates this process. Rapid skin formation can cause the trapped gas within the particle to expand and potentially rupture, leading to disintegration of the particle. Additionally, residual moisture in the particles may lead to the formation of secondary bubbles. The drying kinetics can lead also to other phenomena, such as particle inflation, shrinkage, crater formation, agglomeration, cracks and gaps, as well as particle vacuolation. This stands in stark contrast to the relatively narrow spectrum of morphological features typically observed in agglomerate and crystalline structures, which generally exhibit cracks, occasional crater formation, blowholes, and hollow particles. According to Walton et al. (Walton & Mumford, 1999), the ability to modify structural morphology of skin-forming particles during the drying process is instrumental in achieving extensive diversity and applicability, mainly in food industry. Skin-forming materials include: sodium silicate, sodium dodecyl sulfate (SDS), potassium nitrate, gelatin, skim milk, chicken eggs and maltodextrin (Walton & Mumford, 1999; Zbiciński & Kwapińska, 2003).
Maltodextrin, a water-soluble carbohydrate, plays a prominent role in the food and pharmaceutical industries, offering diverse functionalities, serving as a carrier for flavors and active agents (Sultana et al., 2018), functioning as an emulsifier (Bae & Lee, 2008; Rowe et al., 2006), filler, or substitute for lactose (Hofman et al., 2016). Thanks to its morphological properties, it offers a solution to the degradation challenges frequently faced by powdered products susceptible to caking or stickiness, primarily caused by the presence of low-molecular weight sugars with a low glass transition temperature (Koç & Kaymak‐Ertekin, 2014). Maltodextrin is used as a surface material in microencapsulation of sensitive components, including β-carotene (Loksuwan, 2007), avocado oil (Bae & Lee, 2008) and nutraceutical extracts (Sansone et al., 2011), and serves as an additive to increase the glass transition temperature of products such as honey (Samborska et al., 2015), sumac extract (Caliskan & Nur Dirim, 2013), or strawberry juice (Gong et al., 2018).
An inappropriately carried out drying process can deteriorate the final product’s quality, which is why precise process control is critical for obtaining high quality product. The nutritional and physical properties of food powders, comprising those containing maltodextrin, include aspects such as taste, aroma, color, and particle: agglomeration, density, porosity, dissolution rate, surface properties and size (Anandharamakrishnan & Ishwarya, 2015; Dantas et al., 2023; Hofman et al., 2016; Koç & Kaymak‐Ertekin, 2014). These properties can be altered by manipulating drying parameters, including temperature and flow rate of the drying medium, atomization method, and overall apparatus design, as shown by numerous published studies (Anandharamakrishnan &
Ishwarya, 2015; Caliskan & Nur Dirim, 2013; Dantas et al., 2023; Koç & Kaymak‐Ertekin, 2014; Sobulska et al., 2022; Walton & Mumford, 1999; Zbicinski & Piatkowski, 2009a). In addition, it has been demonstrated (Takeiti et al., 2010) that a significant determinant influencing the particle properties of maltodextrin is the source of the starch hydrolyzed to obtain maltodextrin, along with its dextrose equivalent (DE) value. Recent works on mixtures of maltodextrin with other ingredients, such as proteins or oils, has emphasized the role of maltodextrin concentration in droplet dispersion, which is one of the factors that determine the specification of the resulting powders (Bae & Lee, 2008; Both et al., 2020). Nevertheless, the influence of these parameters is still complex and not sufficiently understood. This research is promising not only for optimizing powder quality but also for improving the energy efficiency of spray drying and encapsulation as well as for exploring potential applications.
Research work performed on a spray drying tower constructed at the Technical University of Lodz has been conducted for an extended period. A substantial body of literature exists that describes the impact of process parameters on the characteristics of the resulting powders in co- and counter-current systems (Zbiciński & Piątkowski, 2004) (Zbicinski et al., 2002), (Kwapińska & Zbiciński, 2005). Additionally, publications described experiments conducted using spray drying, which investigated the quality of products obtained by modifying the process itself. These modifications included the introduction of new parameters, such as foaming the sprayed solution
(Rabaeva & Zbiciński, 2010),(Lewandowski et al., 2019), introducing a swirl of drying air (Wawrzyniak et al., 2020), using two levels of spray nozzles (Wawrzyniak et al., 2024), flame drying (Piatkowski et al., 2014) or conducting a microencapsulation process of oily substances (Lewandowski et al., 2020), (Adamiec & Marciniak, 2006). However, these studies did not fully analyse the level of significance of individual process parameters on the quality of the obtained product, nor did they focus on determining the significance of the effect of the state of the atomised solution on powder formation. What effect does a sudden change in the physical properties of the atomised solution, occurring, for example, as a result of an accident, have on the quality of the obtained product and the course of the drying process?
The research program presented for IFPRI assumes finding the relationship between the rheological properties of the solution and the drying speed on the morphology of the particles obtained by the spray drying method. Therefore, the following tasks were set during the project:
- Selection of suitable experimental media and determination of quality criteria.
- Measurements of rheological properties of aqueous solutions of selected materials.
- Adaptation of the existing equipment to the project requirements.
- Design of particle-free fall SDD measurement system.
- Carrying out spray drying experiments on the semi-industrial scale.
- Analysis of the physicochemical properties of the obtained powder samples.
- Preparation of a monodisperse droplet generator to construct devices to measure drying kinetics.
- Analysis of the influence of the rheological properties and conditions of the spray drying process on the morphology of the melts obtained in the experiments.
- Experiments to determine the temperature of particles during free fall using the IRTUC (InfraRed Temperature for Unknown Coefficients) method.